This invention pertains to a composite membrane for chemical synthesis, a method of using the composite membrane, and a chemical reactor into which the composite membrane is incorporated.
The primary purpose of fuel cells is to generate electrical energy. Certain fuel cells use ion-exchange membrane composites. Chemical energy is converted into electrical energy by reacting different gases at catalytic metal surfaces located on anodes and cathodes which are positioned on opposite sides of the ion exchange membrane. Using hydrogen/oxygen solid electrolyte fuel cells as illustrative, hydrogen is introduced via a first gaseous stream to an anode side of the ion exchange membrane and is electrochemically oxidized in the presence of a suitable catalyst, such as platinum, in accordance with the following general reaction: EQU H.sub.2 .fwdarw.2H.sup.+ +2 electrons.
Protons, liberated during the electrochemical oxidation of the hydrogen, are selectively transported through the ion exchange membrane to a cathode side. Electrons, generated at the anode, are collected and transported to the cathode side via a complex current collector and external circuit system. The collector and external circuit system are constructed of any suitable electrically conductive material such as a stable metal or carbon black.
On the cathode side of the fuel cell, oxygen is introduced by way of a second gaseous stream and is electrochemically reduced in accordance with the following general complete combustion reaction: EQU O.sub.2 +4H.sup.+ +4 electrons.fwdarw.2H.sub.2 O.
However, a partial combustion product, hydrogen peroxide (H.sub.2 O.sub.2), may also form. The equation of this reaction is: EQU O.sub.2 +2H.sup.+ +2 electrons.fwdarw.H.sub.2 O.sub.2.
In fuel cells, where the aim is maximum production of electricity and complete combustion (reduction of oxygen to water), this formation of H.sub.2 O.sub.2 constitutes a particular problem because overall electricity output is reduced.
Most H.sub.2 O.sub.2 is manufactured by a well known anthraquinone process. See, e.g., Binran, 1 Appl. Chem., Ed. Chem. Soc. 302 (Japan 1986). Among the disadvantages of this process are that it requires the addition of numerous organic solvents, forms many unwanted by-products, and requires various separation steps. In contrast, fuel cells provide a potential means for synthesis by safely reacting H.sub.2 and O.sub.2 directly in a single reactor without the use of organic solvents.
Using a reactor cell design similar to fuel cells provides an environment wherein reactants are separated by an ion exchange membrane. With H.sub.2 O.sub.2 synthesis, for example, it is advantageous to separate the H.sub.2 and O.sub.2 reactants because mixtures of the reactants are explosive, especially at higher pressures, and constitute a serious safety hazard. Separating the reactants allows relatively high pressures to be used safely, increasing the mass transfer rate of the reactants. Reactor cells also provide effective environments for the use of catalysts that are optimized for specific reactions.
However, fuel and reactor cells typically require complex electrical equipment in order to collect and transport electrons from one side of the cell to the other. This equipment is generally inappropriate for large scale manufacturing operations. Other methods require the input of external electrical energy and/or the use of corrosion resistant equipment. In addition, many reactor cells require relatively high operating temperatures (e.g. 70.degree. C.-90.degree. C.) in order to be effective. It would be desirable to have a method and reactor cell that does not require organic solvents, complex electrical equipment, or input of external electricity, but yet relieves the danger of explosion and is effective at room temperature.